Why is time measured in seconds in the SI system?

Time is measured in seconds in the SI system because it is the base unit for time in this internationally recognised system.

The International System of Units, or SI, is a globally accepted and used system of measurement. It was developed to provide a standardised and consistent framework for scientific research and communication. The SI system is based on seven base units, one of which is the second, representing time.

The second was chosen as the base unit for time due to its historical use and universal understanding. The concept of the second originated from the division of the day into smaller parts, which was a practice dating back to ancient civilisations. The day was divided into 24 hours, each hour into 60 minutes, and each minute into 60 seconds. This division was widely accepted and used, making the second a logical choice for the base unit of time.

In the modern SI system, the second is defined more precisely using atomic standards. It is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom. This definition allows for a highly accurate and consistent measurement of time, which is crucial in many areas of science and technology.

So, the second is used as the base unit for time in the SI system due to its historical significance, universal understanding, and the ability to define it with high precision using atomic standards.